Implantable pacemakers are capable of sensing the evoked response of a heart and delivering of pacing therapy if the heart fails. In heart failure, as the heart weakens, the morphology of the evoked response changes.
In certain types of heart failure, such as congestive heart failure or CHF, the change in the morphology of the evoked response can occur rapidly, or slowly, over time. Thus, analysis of the morphology and parameters of the evoked response, and the behavior such over time, is useful in predicting heart failure. If heart failure is indicated, treatment such as an alert system, medication delivery for example by an implantable pump, V-V timing control, etc. can be administered. Various devices and methods exist for analyzing the evoked response of a patient's heart as an indicator of cardiac health. Examples are found in European Patent Application EP 1 111 307 A1, by Fishler et al., filed Jan. 18, 2001, entitled AN IMPLANTABLE CARDIAC DEVICE FOR AND METHOD OF MONITORING PROGRESSION OR REGRESSION OF HEART DISEASE, and U.S. application Ser. No. 09/487,858, filed Jan. 1, 2000, both herein incorporated by reference in their entireties.
The morphology of the evoked response, however, also changes in response to patient posture. Thus, the evoked response morphology without considering posture can be unreliable as an indicator of heart failure.
If the evoked response morphology is to be successful as an indicator of heart failure, what is needed is a method and device capable of ensuring the validity of heart failure surrogate parameters from an evoked response.